Currently, the elementary unit of a light-emitting diode (“LED”) light source or lighting pixel for a full-color LED display panel or a single color LED lighting panel is a packaged LED unit. Such packaged LED units are typically mounted onto a printed circuit board (“PCB”) of the display or lighting panel by a surface-mount technology (“SMT”) assembly process. During this process, solder paste is first printed onto solder pads of the PCB, and pre-packaged LED units are then individually placed onto corresponding locations on the PCB by an SMT machine. The PCB on which the individual LED packages have been populated will then be heated in a reflow oven in order to reflow the solder paste underneath the LED packages. This forms strong solder joints between the leads of the LED packages and the solder pads of the PCB.
In the aforementioned approach, the density or lighting resolution of the display or lighting panel is limited by the size of an individual packaged LED unit. The smallest possible red, green and blue (“RGB”) LED pixel assembled from a packaged LED unit is about 0.5 mm by 0.5 mm and the corresponding pitch between lighting pixels is about 0.7 mm to 0.8 mm. Further reduction of the pitch between adjacent lighting pixels is thus not possible if such packaged LED units are used for assembling high density display boards in the above manner.
In order to increase the pixel density further for producing high-density display or lighting panels, an alternative process is to bond bare flip-chip LED dice directly onto bond pads of a PCB of the display or lighting panel. Flip-chip bonding processes are preferred to conventional wire-bonding processes for the realization of the joints at the interconnects of LED dice, since lighting pixel density can be increased using flip-chip bonding processes. The relevant flip-chip LED being bonded may vary in size from 3 mils by 5 mils (75 microns×125 microns) to 6 mils by 10 mils (150 microns×250 microns), and the thickness thereof may be about 3-6 mils (75-150 microns).
FIG. 1 is an isometric view of a typical flip-chip LED 100, showing its geometry and configuration. The flip-chip LED 100 has a pair of bond pads that act as electrodes, namely a p-electrode bond pad 102 and an n-electrode bond pad 104. Supplying current through these electrode bond pads 102, 104 will result in light being emitted from an active light emission layer 106 of the flip-chip LED 100. The p-electrode bond pad 102, n-electrode bond pad 104 and active light emission layer 106 are supported on a substrate 108, which may be made from sapphire, gallium arsenide, or other suitable materials.
FIG. 2 is a schematic electrical circuit diagram 110 illustrating a layout of a typical RGB display board, which may be in the form of a PCB. Such PCBs are fabricated in accordance with a required RGB display board format, and the bond pads on the PCB are configured for receiving the p-electrode bond pads 102 and n-electrode bond pads 104 of the flip-chip LED 100. In FIG. 2, each pixel comprises an RGB set 112 including respective red, green and blue flip-chip LEDs 100. In respect of each flip-chip LED 100, a p-electrode bond pad 102 is electrically connected to a first voltage 114 and an n-electrode bond pad 104 is electrically connected to a second voltage 116. The first voltage 114 is connected to a current source voltage control switching pin of a matrix LED driver circuit. The second voltage 116 is connected to a current sink voltage control switching pin of the matrix LED driver circuit.
For flip-chip LEDs 100 having dimensions of less than 75 microns by 75 microns (3 mils by 3 mils), conventional pick-and-place equipment may be unsuitable for transferring them, and special equipment is needed for the relevant die sorting processes and to mount them onto PCBs during die-attach processes.
The electrodes 102, 104 of the flip-chip LED 100 are typically coated with a layer of gold, wherein a nickel layer may be used as a buffer and barrier layer underneath the said layer of gold. The bonding materials for making the joints between the electrodes of the flip-chip LED and the bond pads of the circuitry on the printed circuit board may comprise: (i) gold bumps or studs, (ii) gold-tin eutectic solder material, (iii) conductive adhesive in the form of epoxy or adhesive paste (such as silver adhesive paste), or (iv) lead-free solder or solder paste such as Sn—Ag—Cu (SAC) solder.
Based on the bonding materials being used, various bonding techniques can be used for bonding the flip-chip LED onto the PCB. For instance, a direct chip-on-board bonding process for flip-chip LED can be realized by thermosonic flip-chip bonding.
In terms of a thermosonic flip-chip bonding process, flip-chip LEDs 100 having gold bumps or studs are individually bonded onto bond pads with nickel-gold (Ni/Au) metallization on a rigid PCB substrate one at a time at elevated temperatures under compression and ultrasonic actuation of the bond head for a given bond time as dictated by such thermosonic flip-chip bonding process. There are high quality requirements in terms of hardness and the prevention of organic or inorganic contamination of the surface finishes of the bond pads on the PCB, on which metallic bonds between the bond pads and the gold bumps or stud are formed under the thermosonic actuation.
Since each flip-chip LED 100 is bonded individually onto the bond pads of the PCB with the utilization of a specific bonding profile, the time taken to complete the bonding of all the flip-chip LEDs required in a modern high-resolution display or lighting panel would be very long and throughput is extremely limited.